1. Field of the Invention
The invention relates to an improved fuel injection system for internal combustion engines.
2. Description of the Prior Art
A fuel injector with two rows of injection nozzles in the form of holes, to each of which an inner nozzle needle and coaxial to it an outer nozzle needle are assigned, is known for instance from German Patent Disclosure DE 102 05 970 A1. Such injection nozzles, which when triggered as a function of pressure open various injection cross sections, are also known as Vario nozzles. The outer and inner nozzle needles are each assigned a respective control piston, and each of these pistons acts on a fuel-filled hydraulic chamber, so that the hydraulic chambers act as actively connected control chambers. The two control chambers communicate hydraulically with one another via a connecting conduit. The control chamber of the outer nozzle needle can be made to communicate with a low-pressure return system via an outlet throttle. The connecting conduit is dimensioned such that upon opening of the outlet throttle, first the pressure in the control chamber of the outer nozzle needle drops, and only after a delay does the pressure in the control chamber of the inner nozzle needle drop.
To increase the injection pressure, which is above the pressure level of the pressure reservoir (common rail), German Patent Disclosure DE 103 29 417 A1 discloses a fuel injection system with a pressure booster device, in which to improve the injection characteristic in addition and to increase the efficiency, a Vario nozzle is likewise employed. The Vario nozzle has two coaxially disposed nozzle needles. The opening pressure of the inner nozzle needle is set either to a constant level with spring support, or to a defined ratio between the rail pressure and the opening pressure with the aid of an additional assisting pressure. As a result, it is possible to adapt the hydraulic flow through the fuel injector to the load point of the engine. The inner nozzle needle is set such that it opens only at relatively high pressures, for instance of greater than 1500 bar, in order to achieve good emissions values in the partial load state of the engine. The setting of the constant opening pressure for the inner nozzle needle is very vulnerable to tolerances, since an abrupt change in the injection quantity occurs upon the opening of the inner nozzle needle. To this extent, variations from one manufactured item to another make themselves especially unpleasantly felt. In the other variant of attaining the opening pressure of the inner nozzle needle via the constant ratio between the assisting pressure and the nozzle pressure also opens the inner nozzle needle even at partial load of the engine.
To prevent the effects of variations in the triggering duration of the control valve on the injection quantity in fuel injection systems with a pressure booster, it has already been proposed in German Patent Disclosure DE 102 29 415.1 that the opening speed of a single nozzle needle be damped, without impairing fast closure of the nozzle needle. A damping piston that defines a damping chamber and that communicates with the closing chamber of the nozzle needle via an overflow conduit is located, axially guided, in the closing chamber of the nozzle needle.